1. Field of the Invention
This invention relates to a digital color encoder for use in color television equipment using a solid stage image pickup such as a charge coupled device.
2. Description of the Prior Art
In the case where image pickup outputs obtained from, for example, a charge coupled device (CCD) are digitally processed to produce a digital color television signal and a rate of three times the color subcarrier frequency f.sub.sc is selected during the modulation process of the digital color signal and the digital modulated color signal corresponding to a chrominance signal is produced in the following process.
In other words, the digital modulated color signal is composed of three vectors a, b and c having phase differences of 2/3.pi. from each other as illustrated in FIG. 1A. Therefore, the red, green and blue primary color signals R, G and B are digitally processed so as to have absolute values of vectors such as U, ((.sqroot.3/2)V-1/2U), and (-(.sqroot.3/2)V-1/2U) as illustrated. Thereafter, they are sequentially switched every 1/3f.sub.sc, which is the processing rate of the color encoder to convert the respective vector components to time series mode. Thus, the digital color signal is amplitude modulated with three phase modulating vectors to produce a desired digital modulated color signal.
The picture elements of the CCD used as the solid state image pickup device are normally arranged linearly in the vertical direction. When a signal of 3f.sub.sc (in general, n/m.times.3f.sub.sc where m and n are relatively small integers) is used as the sampling signal supplied to the CCD, since this frequency is selected to be an odd number times as great as 1/2f.sub.H (f.sub.H is a horizontal frequency), or selected to be 3f.sub.sc =3.times.(455/2)f.sub.H, a 1/2 offset occurs between horizontal lines. Accordingly, if the sampling signal is a continuous wave the relationship between the picture elements and the phase of the sampling pulses is as shown in FIGS. 2A and 2B. The phase of the sampling pulses can be defined as the phase of the read-out clock of the picture signals projected on the picture elements of the CCD chip.
If the relative relation between the picture elements p and the sampling pulses S.sub.p at, for example, N line is as illustrated in FIGS. 2A and 2B, the relative positional relation between the picture elements p and the sampling pulses S.sub.p at N+1 line will be shifted by .pi.. Thus, with this relative positional relationship, it is impossible for the information of the picture elements at N+1 line to be sampled to produce a normal video signal. As a result, it is desirable that the sampling pulses supplied to the respective picture elements of the CCD be reset at every other line so that the sampling phases are in-phase with the picture element arrangement even at N+1 line (FIG. 2C). In other words, it is also desirable that at N line and at N+1 line, the phase of the sampling pulses S.sub.p be arranged to always appear at the relative positions of the respective picture elements 1, 2, 3 . . . .
FIG. 3 is a view illustrating the relative positional relationship among picture elements p of CCD chip 1, the sampling carrier c having a frequency of 3f.sub.sc and the sampling point s. It can be observed that at N line and at N+1 line, the position of the sampling point or the sampling timing s is changed so that at N+1 line the picture of each element will not be picked up.
In the case of sampling the information of the picture elements with phases of sampling pulses which are coincidence between the N and N+1 lines using the resetting operation discussed above as shown in FIG. 2C, when the sampling phase at, for example, N line coincides with the phase of the modulating color vector shown by the solid line in FIG. 1B, since at N+1, the sampling phase is shifted by .pi. relative to the continuous phase of the N line, the phase of the modulating vector at that time will become opposite and the continuity of the sampling phase disappears as is illustrated in FIG. 1B by broken lines. In this case, it can be observed that the phases of the sampling signals of the picture elements at N line and at N+1 line coincide with each other between the respective lines but the continuity of the sampling signals between the respective lines, is inverted by 180.degree.. Accordingly, the phase of the chroma signal modulated by the sampling carrier illustrated in FIG. 2C is different from the phase mode of the color subcarrier signal which is the ideal sampling mode of the NTSC television system so that the desired digital modulated color signal cannot be obtained under the above conditions.
FIGS. 2D and 2G respectively correspond to FIGS. 2C and 2F and show the chroma phase of adjacent horizontal lines in the case when it is assumed that the chroma phase of the same picture elements is 0.degree.. It can be observed that the phase relationship illustrated in FIG. 2G is a so-called offset carrier of 0.degree. and 180.degree. of the NTSC space TV signal.